Multi-phase fluids typically comprise both gas and liquid components and an example would be a well stream extracted from a topside or sub-sea well which comprises a mixture of gas, oil, water and some salt components. Such a mixture can vary substantially as regards the proportions of its gas and liquid components. In the context of this invention the term “fluid” includes a liquid-phase alone (including multiple liquid phase fluids, e.g. a mixture of oil and water), a gas-phase alone, or a combination of gas and liquid phases.
Multi-phase mixtures with a very high gas volume fraction (GVF) are known as condensate or “Wet Gas”—a geological term for a gaseous mixture of hydrocarbons that contain a significant amount of compounds with molecular weights heavier than methane. Such wet gas fluids typically have a GVF of above approximately 95% corresponding to a gas liquid ratio (GLR) above 20. Typically such fluids also contain other non-hydrocarbon compounds such as carbon dioxide, hydrogen sulphide, nitrogen, oxygen and water.
One arrangement for sampling fluids from a hydrocarbon well-bore is known from U.S. Pat. No. 6,435,279 in which a multi-phase fluid is collected from a well-bore using a self-propelled underwater vehicle. The vehicle comprises a collection device and a storage device, and is arranged to collect a sample of the multi-phase fluid from the well-bore and store the collected fluid in the storage device. The vehicle is then moved to a location where the collected sample of fluid can be recovered to enable measurements to be performed on the collected sample.
A drawback with this known arrangement is that the pressure and/or temperature are not maintained between taking a sample from the sample point on the well bore and testing the sample at the remote location, i.e. isobaric and isothermic conditions are not necessarily maintained. This may compromise the sample quality, i.e. representativeness of each phase, and limit the usefulness of the sample. Thus, the gas/liquid ratio at the sample measurement location may not be representative of the gas/liquid ratio at the sample extraction location. Furthermore, phase transitions may occur and/or mass transfer between the phases, resulting in samples that are not compositionally representative. It is preferable that the sampling process does not alter the phase composition.
Samples for the purpose of updating pressure-volume-temperature data for a multiphase flow-meter have to be taken in close proximity to a multiphase flow-meter, otherwise fluid and gas composition of a sampled fluid might be different due to different pressure and temperature elsewhere in a pipeline, and possible release of some of the gas present inside the oil at higher pressure.
There may be several reasons for obtaining a multiphase or wet gas sample, such as: (i) to reduce multiphase metering uncertainty; (ii) for reservoir and production management; (iii) to obtain well chemistry and flow assurance information per well; and (iv) to obtain information for subsea processing.
The invention seeks to provide for a conduit, related method and system offering advantages over known such conduits, methods and systems, and provides an improved conduit, method and system for sampling multiphase fluids.